Photodissociation in the atmosphere: 1. Actinic flux and the effects of ground reflections and clouds

TLDR: In this paper, it was shown that photodissociation rate coefficients inside clouds, and particularly inside cloud droplets, can frequently exceed the clear-sky values, in contrast to current usage in cloud chemistry models, due to the ∼2 cos θ factor incurred in the actinic flux when the solar beam is scattered and diffused into nearly isotropic light.

Abstract: The actinic flux must be distinguished from other radiometric quantities such as the irradiance. This distinction shows that (1) a fraction of the recent atmospheric chemistry literature contains improperly calculated rates of photodissociation, and (2) photodissociation rate coefficients inside clouds, and particularly inside cloud droplets, can frequently exceed the clear-sky values, in contrast to current usage in cloud chemistry models. Both of these findings are traceable to the ∼2 cos θ factor incurred in the actinic flux (but not in the irradiance) when the solar beam is scattered and diffused into nearly isotropic light.